1. Field of the Invention
The present invention relates to an optical apparatus, such as an exposure apparatus, having a section that is purged by, for example, inactive gas in at least a part of an optical path leading through a reticle or the like from a light source to an object to be exposed, and also relates to a concentration measuring mechanism that measures the concentration of a certain gas contained in the inactive gas.
2. Description of the Related Art
Recently, integration density of semiconductor integrated circuits has increased, so that nano-order micromachining is required at present. For micromachining, exposure apparatuses are designed to have high cleanliness for stable exposure performance and are used in a clean environment such as a clean room, and the wavelength of light from a laser serving as an exposure light source is reduced. With the decrease in wavelength, however, exposure light causes impurities inside the exposure apparatus to photochemically react with oxygen (O2), and a photoproduct produced by this photochemical reaction adheres to optical elements (e.g., lenses and mirrors). This fogs the lenses and mirrors, and reduces the illuminance.
In conventional exposure apparatuses using a KrF or ArF excimer laser as a light source, optical elements arranged on a laser optical path are housed in a space purged by inactive gas in order to prevent the illuminance from being reduced by the adhesion of the impurities to the lenses and mirrors, and to prevent a decrease in transmittance due to optical absorption by, for example, oxygen contained in the atmosphere in the optical path. Moreover, by monitoring the oxygen concentration of the interior, the degree of purging of the interior is detected, and it is determined whether exposure environment conditions are satisfied.
FIG. 8 is a schematic view of a part of an example of an exposure apparatus. A structure 101 serving as part of an optical system of the exposure apparatus has a lens unit 102 therein. Exposure light 106 is guided from a laser light source 105 to the outside of the structure 101 through sealing glass plates 103. Inactive gas is supplied into the structure 101 from one of pipes 107a and 107b, and is exhausted from the other pipe so that the interior of the structure 101 is constantly purged by clean inactive gas. A rotary member 108 has filters 109a and 109b for controlling the exposure light 106, and is driven by a motor 110. The motor 110 is fixed to the exposure apparatus by a motor holder 111. In this configuration, when the above-described photoproducts produced by photochemical reaction adhere to and fog the lens unit 102 and the light control filters 109a and 109b, the structure 101 is often opened to the atmosphere in order to replace the lens unit 102 and the light control filters 109a and 109b with new ones and to change the light control filters 109a and 109b to various types of filters for adjustment of the output of the exposure light. Therefore, the oxygen concentration inside the structure 101 frequently increases.
In the exposure apparatus, optical cleaning is performed to remove dirt from the optical elements. In optical cleaning, ozone is produced by injecting oxygen into the space occupied by the optical path, and the dirt is removed by the action of the ozone. After this maintenance operation, the oxygen concentration inside the structure 101 also increases.
Accordingly, an oximeter (oximeter) 112 shown in FIG. 8 or an oximeter 113 shown in FIG. 9 is provided to measure the oxygen concentration. By measuring the oxygen concentration inside the structure 101, it is determined whether the atmosphere inside the structure 101 is sufficiently replaced with inactive gas and does not have any influence on exposure performance. Exposure is started after the structure 101 is opened to the atmosphere based on this determination.
For example, Japanese Patent Laid-Open No. 11-087230 discloses an exposure apparatus in which stable exposure performance is maintained by monitoring the oxygen concentration with an oximeter provided inside the exposure apparatus.
Of the short-wavelength laser light used in the conventional exposure apparatuses, ArF laser light, which is absorbed by oxygen, provides a sufficient transmittance at an oxygen concentration of approximately 50 ppm to 100 ppm. For an oximeter that measures the oxygen concentration within the above range, the required measurement accuracy is approximately ±10 ppm to ±50 ppm. That is, in the conventional exposure apparatuses, since there is no need to measure a low oxygen concentration, the measurement is not taken into consideration. Therefore, even when the oximeter is exposed to the atmosphere containing a large amount of oxygen, since the measurement error of the oximeter is originally large, a phenomenon in which the measurement accuracy is decreased when the oximeter is exposed to the atmosphere containing oxygen having a concentration higher than the upper limit of the measurable concentration (hereinafter referred to as a high-concentration shock) does not substantially influence the measurement error. Therefore, it is unnecessary to consider the high-concentration shock.
However, the absorptance of oxygen for F2 laser light that is used in next-generation exposure apparatuses is more than or equal to 100 times the absorptance for ArF laser light. F2 laser light is absorbed not only by oxygen, but also by moisture. Therefore, in order to obtain an illuminance equivalent to that of ArF laser light, both the oxygen concentration and moisture concentration on the laser optical path in the exposure apparatus must be less than approximately 10 ppm, and must be managed and maintained at extremely low values. For that purpose, it is necessary to measure a low concentration with high precision (e.g., on the order of 0.1 ppm). However, as the performance of the concentration meter that can measure a low concentration increases, the concentration meter undergoes a greater high-concentration shock, and requires a long recovery time to be ready for precise measurement.
FIG. 7 is a graph that compares experimental values (broken line) obtained by measuring the oxygen concentration inside a chamber purged by inactive gas and calculated values (one-dot chain line) of the oxygen concentration inside the chamber. A chamber purged by inactive gas was opened for several minutes and was purged again, and the oxygen concentration of the atmosphere in the chamber was then measured. In a conventional method, the chamber was opened while an oximeter was left therein, and the measurement was taken after purging was started again. As shown in FIG. 7, when the exposure apparatus is opened for maintenance and filter replacement, the oximeter exposed to the atmosphere undergoes a high-concentration shock, and takes a long time to indicate precise measurement values. In some cases, the oximeter may break down.
For example, when it is assumed that exposure is started at a point A in FIG. 7, since the oximeter has not recovered yet from the high-concentration shock in the conventional measurement method, it shows a value higher than the actual concentration. Therefore, it is erroneously determined that the oxygen concentration has not been sufficiently reduced yet, and exposure cannot be started. Consequently, the downtime of the exposure apparatus is longer than necessary, and the exposure apparatus is stopped for a long period.